There are prior art rotational screws for mixing. For example, a barrier type is disclosed in U.S. Pat. No. 3,486,192 (see FIG. 6) and a dam-flight type is disclosed in Japanese Unexamined Patent Publication No. 11505/1967 (See FIG. 7).
The barrier type screw 60, in FIG. 6, has a mixing portion 62 thereof which is equal to flight portions thereof in diameter and provided on the central region of a screw shaft 61. On the periphery of the mixing portion 62 there are a plurality of axially extending mixing recesses 63. In operation with this barrier type screw 60, while material supplied into the mixing recesses 63 is transported in the axial direction as the screw shaft 61 rotates, a portion of material moves in a direction opposite to the rotating direction of the screw shaft 61 and will flow into the nearest recess 63 situated in a circumferential direction. This causes the material to flow disorderly in two directions on the mixing portion 62 to thereby prevent uniform mixing with additives.
The dam-flight type screw 70, in FIG. 7, has a lead of main flight 72 disposed on a screw shaft 71 thereof and also, a lead of auxiliary flight 73 having a large pitch. In addition, a mixing portion 74 of the screw 70 is formed such that the distance between the flights 72 and 73 becomes narrow in the feeding direction. Thus, as the screw shaft 71 rotates, a portion of material moves across the auxiliary flight 73 and is joined with the flow of material moving along the back side of the auxiliary flight 73. However, this action does not result in continuous and appropriate mixing of the material; rather, it results only in agitation of the same with both of the flights 72 and 73. Thus, it is difficult for the arrangement to provide a uniform mixture of material constantly by intimately mixing the material with additives.